The reason is progression. We have been working very hard to make sure we design the abilities system to offer smooth progression into ME3 from ME2. Allowing double progression on characters makes that almost impossible. We have to have reasonable knowledge about where a character could end up finishing all content on a playthrough. We also want to offer choice of character build within specific classes.

We replaced that feature with playing after you were done because it preserves progression and allows for smoother downloading of PRC.